1. Field of the Invention
The present invention relates to an optical disc, an apparatus for reproducing the same and a method of recording and reproducing the same.
2. Description of the Related Art
There is an optical disc for recording and reproducing audio information and video information. The principle of recording and reproducing operation of such an optical disc is explained with referring to FIG. 11.
In FIG. 11, upon recording audio information and/or video information onto an optical disc 92, the information is converted to a pit length and an inter space between information pits 91 on an information recording surface 95 of the optical disc 92, to be recorded. On the other hand, upon reproducing the audio and/or video information from the optical disc 92, a laser beam having a laser beam spot LS with a predetermined diameter is irradiated onto the information pits 91, and the reflected and diffracted light is detected by a light detecting device composed of a photodiode etc., to convert it to an electrical signal. Then, a signal conversion reverse to that applied at the time of recording is applied to the electrical signal and the audio and/or video information is reproduced.
As an optical disc used in this manner, there are a compact disc (CD), a laser vision disc (LVD) and so on. In FIG. 11, a reference numeral 93 represents a substrate composed of polycarbonate resin etc., and a reference numeral 94 represents a protection layer.
The information recording density of this kind of optical disc, is greatly influenced by the value of a track pitch P1 shown in FIG. 12, and the value of the diameter of the laser beam spot LS. In order to record more information on an optical disc, it may be tried to reduce the track pitch or reduce the diameter of the laser beam spot LS. However, a practically successful result of such a trail is not reported, so far.
As mentioned above, since it is rather difficult to increase the information recording density by reducing the track pitch or reducing the laser beam spot, it may be proposed a method to increase the information recording density by irradiating a laser beam onto a pit which has a plurality of different shapes, and reading the distribution of the reflected and diffracted light from the pit by a photodetector. This method aims at increasing the information recording density by increasing the information amount per one pit, from one bit (i.e. the presence and absence of the pit) to n bits. However, this method has a problem that the shape of the pit cannot be correctly detected often due to the influence of the shapes of the pits in the vicinity of the pit to be read, and the influence of the offset or shift between the center of the information unit and the center of the laser beam spot.